Hitherto, small thermo-setting resin articles have been injection moulded from resin based compositions utilizing heated dies and this has proved satisfactory; but when extensive sheets are required the mould costs associated with conventional injection moulding techniques become prohibitive. This is primarily due to the high temperatures and pressures involved (at least several hundreds of degrees centigrade and several thousands of pounds per square inch) and the need to maintain dimensional accuracy in the thickness of the finished sheet throughout its entire longitudinal and transverse extent. When long rods are required a similar difficulty arises. Indeed the problems of such that injection moulding is avoided and such articles are either not used or are fabricated by other techniques.
One widely adopted expedient for the production of sheets is to utilise a chemically reactive two part resin base and to float a layer of the uncured composition onto a moving conveyor for its support until it sets. The conveyor is usually vibrated to settle the layer and hasten the migration of air bubbles in the composition to the upper surface. Two major disabilities arise. Firstly, the long setting time requires either a long, and therefore expensive, conveyor or a very slowly moving conveyor with a consequent low production rate and secondly a finish machining operation is required to obtain dimensional accuracy and to eliminate air bubbles in the surface layer of the sheet. The latter is particularly objectionable because it is inherently wasteful of a very expensive raw material.